Radiation polymerizable compounds and conductive coatings from same

ABSTRACT

This invention is directed to a novel water soluble conductive material which is capable of being polymerized by actinic radiation. More specifically, this invention involves the use of a radiation curable quaternary ammonium salt obtained by mixing at least one tertiary amine having acryloyl or methacryloyl unsaturation and at least one halide. When the water-soluble conductive material is subjected to actinic radiation sufficient to cause polymerization, it maintains its conductive character, but becomes essentially impervious to attack by water or solvent, and remains tack-free even at high relative humidity. When the water-soluble conductive material is polymerized by actinic radiation, it is not necessary to drive water off during cure and then rehumidify to obtain proper conductivity as is often necessary with the prior art coatings.

FIELD OF THE INVENTION

This invention is directed to a novel water soluble conductivequaternary ammonium salt capable of being polymerized by actinicradiation and to a process for coating a substrate with same.

BACKGROUND

Conductive coatings are useful in a number of areas such as antistaticcoatings, shielding applications, photo-imagable coatings and inphotocopying applications. Although a number of conductive coatings areknown in the prior art, there are limitations associated with their use.

One commercial type of conductive coating involves the use of highmolecular weight, solvent-borne coatings which are not subsequentlycured. Although these systems have good conductive characteristics, theyare still subject to attack by solvent. For instance, if these coatingsare used in a photo-imagable application, the electroconductive layer istypically topcoated with a dielectric layer. When both the dielectricand the conductive layer are coated from the same type of solvent, it isdifficult to prevent the extraction of at least a small amount of theconductive material into the dielectric material thereby decreasing itseffectiveness.

Radiation curable conductive materials are known in the art but thesealso have certain limitations. Prior to application, these coatingstypically require the addition of free acrylic monomers for viscosityreduction. This is especially true when the polymer itself is an acrylicwhich has been prepared by free radical polymerization. The diluentacrylic monomers are often volatile and may contribute a toxicity hazardto the handling of the materials.

Many of the water-reducible coatings available in the prior art arerelatively high in molecular weight and, therefore, must be reduced toless than about 50% solids for application. The excess water remainingin the coating must be driven off by heating, thereby using additionalenergy.

These problems are overcome in the present invention by using a watersoluble, low molecular weight quaternary ammonium salt, which can bepolymerized by subjecting it to actinic radiation. Since the product ofthis invention is a low molecular weight, water-soluble salt, lowapplication viscosities can be achieved at high solids without the useof acrylic monomers and by using water as the only solvent. Once thesalt of this invention has been cured by heat or actinic radiation, itis essentially impervious to attack by water or organic solvents.

SUMMARY

It is an object of this invention to provide a polymerizable,water-soluble quaternary ammonium salt. It is a further object of thisinvention to provide a crosslinked conductive coating by subjecting anovel water-soluble quaternary ammonium salt to heat or actinicradiation sufficient to polymerize the salt through its unsaturation. Itis a further object of this invention to provide a radiationpolymerizable quaternary ammonium salt which can be applied at highsolids using water as the only solvent. It is a further object of thisinvention to provide a radiation polymerizable conductive material whichcan be polymerized by ultraviolet or electron beam actinic radiationwithout the use of added sensitizers or reaction promoters. It is afurther object of this invention to provide a radiation curable productwhich does not require the addition of acrylic monomers for viscositycontrol.

These and other objects of this invention will be apparent from thefollowing description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention teaches a novel composition of matter and a process fortreating a substrate with a conductive coating. The invention taughtherein involves a radiation crosslinkable coating which comprises anaqueous solution of a quaternary ammonium salt wherein the salt isobtained by mixing reactive amounts of: (a) at least one tertiary amineof the formula: ##STR1## wherein R₁ is H or CH₃, Z is 0 or NH, R₂ is analiphatic radical of from 1 to 6 carbon atoms or aryl of from 6 to 10carbon atoms, R₃ and R₄ are the same or different and are selected fromthe group of aliphatic radicals containing from 1 to 10 carbon atoms andaryl radicals containing 6 to 10 carbon atoms; and (b) at least onehalide having from 2 to 6 halogen atoms and selected from the groupconsisting of aliphatic halides having from 1 to 10 carbon atoms andarene halides having from 7 to 20 carbon atoms.

For purposes of this disclosure, the term reactive amounts means betweenabout 0.8 to 1.5 equivalents of the tertiary amine is available for eachequivalent of halogen. It is especially preferred that about oneequivalent of tertiary amine be available for each equivalent ofhalogen. When an excess of equivalents of either the amine or the halideis used to drive the reaction to completion, the excess unreactedmaterial is easily separated out because it is not as water soluble asthe quaternary ammonium salt product. Typically, unreacted materials canbe readily removed based upon this difference in solubility.

Upon mixing, the reaction between the tertiary amine and the halide isvery thermodynamically favorable and it rapidly exotherms. Therefore, itis generally preferred to add one of the reactants very slowly to theother with constant stirring while maintaining the reaction attemperatures between about 20° to 100° C. Typical reaction times forproducing significant amounts of the desired salt, range from about fiveminutes up to several hours depending on the reaction temperature andthe reactivity of the components.

A representative example of the reaction taught herein (using 2 moles of2-(N,N-dimethylamino)ethylacrylate and 1 mole of 1,4-dichlorobutane forillustrative purposes) would be: ##STR2##

Since one of the major advantages of this invention is the ability toapply the aqueous solution of the salt at very high solids, it isdesirable to keep the molecular weight of the salt, and, therefore, theviscosity of its aqueous solution, as low as possible. Therefore, it issometimes advisable to minimize any possibility of polymerizationthrough the double bonds during the course of the manufacture of thequaternary ammonium salt by using techniques well known in the art tominimize free radical polymerization, e.g. by shielding the reactionvessel from light, by passing a stream of oxygen through the reactionmixture or by the use of stabilizers such as hydroquinone and the like.

The quaternary ammonium salts produced by this invention have amolecular weight less than about 5000 and preferably have a molecularweight less than about 1000 and most preferably have a molecular weightless than about 500.

For ease of handling, it is preferred that the tertiary amine and thehalide be mixed in the presence of water so that the salt will besolubilized as soon as it is produced. The exact amount of water used inthe aqueous coating of this invention is not critical as long as thereis sufficient water to solubilize the salt. However, it is moreeconomically favorable to apply the coating at high solids, therefore,it is preferred that the final coating contain not more than about 50%by weight water and it is especially preferred that the coating containnot more than about 25% water. Since the quaternary ammonium saltsproduced by this invention are low molecular weight molecules, lowapplication viscosities, e.g. typically less than 500 centipoise, arecommon for these salts even as a 75% solution in water.

The tertiary amines which are useful in the practice of this inventionare those which have acryloyl or methacryloyl unsaturation therebyproviding the sites for polymerization when the salt is subjected toactinic radiation. Representative examples of useful tertiary aminesinclude the acrylates, such as 2-(N,N-dimethylamino)ethylacrylate,2-(N,N-diethylamino)ethylacrylate, 2-(N,N-dimethylamino)propylacrylateand the like; the methacrylates such as2-(N,N-dimethylamino)ethylmethacrylate,2-(N,N-diethylamino)ethylmethacrylate and the like; and the acrylamides,such as 2-(N,N-diethylamino)ethylacrylamide and2-(N,N-dimethylamino)ethylmethacrylamide and the like.

The halides which are useful in the practice of this invention are thosewhich readily undergo nucleophillic substitution reactions. Especiallypreferred in the practice of this invention are primary halides becausethey are the most reactive with tertiary amines.

Representative examples of the preferred halides include unsaturatedmolecules such as 1,4-dichloro-2-butene, 1,6-dichloro-3-hexene and thelike; saturated molecules such as 1,2-dichloroethane,1,3-dichloropropane, 1-bromo-3-chloropropane, 1,4-dibromobutane,1,4-dichlorobutane and the like; and arene halides such as orthoxylylenechloride and the like. As used herein, the term arene halide means thosemolecules having halogen functional aliphatic groups as substituents onan aromatic ring.

It is believed that under proper reaction conditions, similar materialsto those taught in this invention could be prepared by using an acryloylor methacryloyl functional halide, such as 2-chloro ethylacrylate, alongwith a tertiary amine which could also contain acryloyl or methacryloylunsaturation.

The quaternary ammonium salt prepared within the teaching of thisinvention should contain an average of at least one and preferably atleast two acryloyl or methacryloyl sites per molecule to provideadequate reaction sites for crosslinking and cure of the coating when itis subjected to actinic radiation.

Various pigments and additives well known in the art could also be usedin conjunction with the radiation curable material taught in thisinvention. For instance, if used as photo-imagable coatings there can beused in conjunction therewith conventional pigments and dyes. If thematerial of this invention is used in a shielding application, metallicpowders such as silver or copper could be incorporated therein. Ifdesired, the compositions taught in this invention could also berendered curable by heat as well as by radiation by incorporatingperoxides, azo-catalysts or other heat-activated free radicalinitiators. Other watersoluble resins such as polyesters, acrylics,epoxies and the like could also be incorporated in order to adjust theproperties of the final product.

Although it is possible to polymerize the quaternary ammonium saltstaught in this invention by subjecting them to heat in the presence ofheat-sensitive free radical initiators, the preferred method of cure isby subjecting the unsaturated salts to actinic radiation for a period oftime sufficient to accomplish the desired amount of free radicalpolymerization. Especially useful in the practice of this invention isactinic radiation having a wavelength less than about 4,000 Angstromssuch as ultraviolet light or electron beam radiation. Useful doses ofultraviolet radiation range from about 5 to about 150 feet/minute/lampwhile useful doses of electron beam radiation range from about 0.5 toabout 15 megarads. The materials of this invention show such rapidpolymerization when subjected to actinic radiation that it is generallynot necessary to incorporate a special sensitizer or initiator into theformulation.

Any substrate which would provide suitable adhesion when used incombination with the radiation curable quaternary ammonium salt of thisinvention could be treated with the aqueous coating of the salt toimpart conductive characteristics. Useful substrates include, forexample, paper, cloth, synthetic fibers, paperboard, wood, rubber,metal, glass and plastic films.

The quaternary ammonium salts taught in this invention can be applied byany of the conventional means known in the art. These applicationmethods include spray, reverse roll, gravure, air knife, knife coater,trailing blade, curtain coater, etc. The exact coating weight which isapplied will depend upon the end use, the substrate itself, the curingtechnique and the end use of the polymerized product.

The following examples are presented for the purpose of illustrating theinvention and are not to be construed as limiting its scope.

EXAMPLE 1

A 250 milliliter erlenmeyer flask equipped with a stirring bar wascharged with 82.39 grams (0.482 moles) 2-(N,N-diethylamino)ethylacrylateand 37.51 grams deionized water. As the solution was stirred, 30.11grams (0.241 moles) 1,4-dichloro-2-butene was added to the flask bydropping funnel over a period of 40 minutes. The reactants were stirredand heated to about 160° F. over a period of about three hours. Theproduct was placed into a separatory funnel where it separated cleanlyinto two clear layers. The small upper phase was identified by infraredanalysis as unreacted 2-(N,N-diethylamino)ethylacrylate. The much largerlower viscous phase was the desired quaternary ammonium salt as a 75%solution in water.

EXAMPLE 2

A flask equipped as in Example 1 was charged with 72.56 grams (0.507moles) of 2-(N,N-dimethylamino)ethylacrylate and 37.50 grams ofdeionized water. To this was added, by dropping funnel, 39.94 grams(0.254 moles) of 1-bromo-3-chloropropane over about a 90 minute period.The mixture was stirred and gradually heated to about 185° F. over aperiod of two hours and twenty minutes. The reaction mixture wastransferred to a separatory funnel and the mixture cleanly separatedinto a large viscous light yellow upper layer and a very small lowerclear layer. Infrared analyses indicated the viscous portion was thedesired aqueous solution of the ammonium salt and the colorless lowerlayer was a reaction product of 2-(N,N-dimethylamino)ethylacrylate.

EXAMPLE 3

A flask such as that in Example 1 was charged with 87.1 grams (0.609moles) 2-(N,N-dimethylamino)ethylacrylate and 37.5 grams deionizedwater. By dropping funnel, 25.4 grams (0.203 moles)1,4-dichloro-2-butene was admixed as shown in Example 1. The reactantswere stirred without additional heating for a period of about 61/2hours. As in Examples 1 and 2, the desired aqueous solution of thequaternary ammonium salt was obtained by separating the aqueous phasecontaining the salt from the organic phase containing the impurities andunreacted starting materials by use of a separatory funnel. The aqueoussolution of the salt was a viscous light yellow liquid.

EXAMPLE 4

A 1,000 milliliter flask equipped with a stirring bar and a tube toallow continuous purging with oxygen was charged with 340.1 grams (2.64moles) 2-(N,N-dimethylamino)ethylmethacrylate and 150.0 grams deionizedwater. While oxygen was bubbled through the solution, 109.9 grams (0.88moles) of 1,4-dichloro-2-butene was added to the reaction mixture via adropping funnel over a period of about 21/2 hours. The reactionexothermed to approximately 120° F. and was stirred in the presence ofoxygen for a period of about six hours. The products were placed in aseparatory funnel and the viscous light yellow aqueous layer containingthe quaternary ammonium salt was separated off. Stability samples ofthis methacryloyl unsaturated salt have shown no tendency to polymerizein the absence of actinic radiation when stored for periods of sixmonths or more, but they cure very quickly to give a crosslinked coatingwith excellent conductivity properties when subjected to electron beamor ultraviolet radiation.

The 75% solid aqueous salt solutions prepared in Examples 1 through 4were drawn down over commercially available base paper stock, subjectedto various levels of radiation and then tested for cure and for surfaceelectrical resistivity (SER). One test for cure is to measure thedifference in log SER between the coated side and the uncoated side ofthe base paper. A summary of these test results is given in Table I.

                                      TABLE I                                     __________________________________________________________________________    Effect of Radiation on Curing as Measured by                                  SER* Difference, Front to Back                                                                  Log Surface Electrical Resistivity                                     Average                                                                              No Radiation                                                                             U. V. Cure.sup.2                                                                         Electron Beam Cure.sup.3              Example Number                                                                           Coat Weight.sup.1                                                                    Front                                                                             Back                                                                              Δ                                                                          Front                                                                             Back                                                                              Δ                                                                          Front                                                                             Back                                                                              Δ                       __________________________________________________________________________    1          5.86   8.32                                                                              8.43                                                                              0.11                                                                             6.89                                                                              8.00                                                                              1.11                                                                             --  --  --                            2          10.55  7.59                                                                              7.70                                                                              0.11                                                                             6.59                                                                              7.05                                                                              0.46                                                                             --  --  --                            3          4.66   6.97                                                                              7.89                                                                              0.92                                                                             6.46                                                                              8.59                                                                              2.13                                                                             6.80                                                                              8.55                                                                              1.75                          4          4.95   7.43                                                                              7.89                                                                              0.46                                                                             6.92                                                                              8.66                                                                              1.74                                                                             --  --  --                            Commercial Product 1.sup.4                                                               4.01   9.05                                                                              9.05                                                                              0.00                                                                             7.43                                                                              8.68                                                                              1.25                                     Commercial Product 2.sup.5                                                               6.16   6.06                                                                              7.78                                                                              1.71                                                                             --  --  -- --  --  --                            Commercial Product 3.sup.6                                                               2.61   6.51                                                                              8.38                                                                              1.87                                                                             --  --  -- --  --  --                            Uncoated.sup.7                                                                           --     10.22                                                                             10.22                                                                             0  --  --  -- --  --  --                            __________________________________________________________________________     *Surface Electrical Resistivity (reciprocal of conductivity)                  .sup.1 lbs/3000 ft.sup.2                                                      .sup.2 20 ft/min/lamp                                                         .sup.3 40 megarads                                                            .sup.4 DMS Salt of Dimethylaminoethylacrylate (radiation polymerizable        into a "linear" conductive polymer)                                           .sup.5 Conventional conductive agent polyvinylbenzyltrimethylammonium         chloride                                                                      .sup.6 Conventional quaternary ammonium polymer                               .sup.7 Uncoated commercially available base paper                        

As shown in Table I, the low molecular weight, low viscosity coatingsdescribed in this invention show only a relatively slight difference inlog SER between the front and back sides of the sheet if they have notbeen subjected to radiation. This is because the low molecular weightmaterials which have not been crosslinked can soak through the entirepaper so that there is essentially no difference in SER between thefront side and the back side of the coated sheet. Once the materials ofthis invention are subjected to high energy by ultraviolet or byelectron beam radiation, however, there is a significant increase in thedifference in resistivity due to the fact that as the materialscrosslink through the acryloyl or methacryloyl unsaturation and becomehigher molecular weight, they are less able to penetrate the base paper.In this test, the materials of this invention showed a significantdifference of between about 0.35 and 1.3 decades of resistivity betweenthe coated and uncoated sides when they were subjected to actinicradiation immediately after application. Table I also shows that theprocuts of this invention have SER values within the range of thecurrently available commercial products used in photo-imagingapplications.

Coated paper samples were also subjected to water extraction tests todetermine degree of cure. The coated paper samples were subjected tovarious levels of radiation, placed in a constant humidity room for twodays and then weighed. The coated paper samples were attached to clipsto prevent curling and were placed in a beaker of water which wasvigorously stirred for five minutes. The samples were returned to theconstant humidity room for two days and then reweighed to determinecoating loss. A summary of these results is shown in Table II.

                  TABLE II                                                        ______________________________________                                        Determination of Cure by Water Extraction.sup.1                                            Curing    Coating   % Coating                                    Example Number                                                                             Method.sup.2                                                                            Weight.sup.3                                                                            Weight Lost                                  ______________________________________                                        1            None      5.04      49.9                                         1            U.V.      5.04      26.2                                         3            None      5.04      43.2                                         3            U.V.      5.04      4.3                                          3            E.B.      5.04      5.3                                          4            None      4.95      34.4                                         4            U.V.      4.95      4.2                                          Commercial Product 1.sup.4                                                                 None      4.01      62.6                                         Commercial Product 1                                                                       U.V.      4.01      73.3                                         Commercial Product 2                                                                       None      8.87      79.3                                         Commercial Product 3                                                                       None      2.61      69.9                                         ______________________________________                                         .sup.1 Gravimetric determination of weight loss after 5 minutes immersion     in 25° C. water.                                                       .sup.2 U.V. cures were 20 ft/min/lamp Electron beam cures were at 40          megarads.                                                                     .sup.3 lbs/3000 ft..sup.2                                                     .sup.4 Identity of the commercial products is same as given in Table I.  

As clearly shown in Table II, the materials of this invention show asignificant decrease in the amount of material extractable by waterafter they have been cured by ultraviolet or electron beam radiation.The current commercial products, however, are still subject to a highdegree of water solubility. Even the commercially available DMS salt ofdimethylaminoethylacrylate was still very water soluble even afterpolymerization by U.V. This is because that material is polymerizableonly into a linear conductive polymer. The materials of this invention,however, will typically always contain an average of more than oneacryloyl or methacryloyl sites per molecule which enables these productsto crosslink thereby dramatically decreasing their solubility.

The experimental results outlined in Table I and II also clearlyindicate that the materials of this invention have excellentconductivity characteristics as well as excellent cure when subjected toactinic radiation. The cure coatings also remain essentially tack-freeeven in high relative humidity, apparently due to the high degree ofpolymerization.

While this invention has been described by a specific number ofembodiments, it is obvious that other variations and modifications maybe made without departing from the spirit and scope of the invention asset forth in the appended claims.

The invention claimed is:
 1. A radiation crosslinkable coating whichcomprises an aqueous solution of a quaternary ammonium salt wherein thesalt is obtained by mixing reactive amounts of:(a) at least one tertiaryamine of the formula: ##STR3## wherein R₁ is H or CH₃, R₂ is analiphatic radical of from 1 to 6 carbon atoms of aryl of from 6 to 10carbon atoms, R₃ and R₄ are the same or different and are selected fromthe group of aliphatic radicals containing from 1 to 10 carbon atoms andaryl radicals containing from 6 to 10 carbon atoms; and (b) at least onehalide having from 2 to 6 halogen atoms and selected from the groupconsisting of aliphatic halides having from 1 to 10 carbon atoms andarene halides having from 7 to 20 carbon atoms.
 2. The radiationcrosslinkable coating of claim 1 further characterized in that thehalide has at least two primary halogens.
 3. The radiation crosslinkablecoating of claim 1 further characterized in that the quaternary ammoniumsalt has a number average molecular weight less than about
 5000. 4. Theradiation crosslinkable coating of claim 1 further characterized in thatthe reactive amounts of the tertiary amine and the halide are mixed inthe presence of water.
 5. The radiation crosslinkable coating of claim 1further characterized in that it contains not more than about 50% byweight water.
 6. The radiation crosslinkable coating of claim 5 furthercharacterized in that it contains not more than about 25% by weightwater.
 7. The radiation crosslinkable coating of claim 1 furthercharacterized in that the tertiary amine and the halide are present in achemical equivalent ratio of about 1 to 1.